My personal opinion is that most "ambient" produced by Metal musicians is garbage and pretty superficial. There are exceptions of course, but it's harder than it seems to pull of something decent.

Agreed.
Good recommendations.

Brian Williams basically invented Dark Ambient.

Newer Inade is also great but less dark and cold, somehow more "cosmic".

Iīd add Bad Sector, not the darkest as it sounds strangely emotional and melodic at times (in a great way). Also older Daniel Menche releases, more of an experimental touch there._________________A nobody in a world of somebodies.
A little debil ainīt enough.
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